Typically, an input unit of a programmable controller comprises input circuits incorporating photo-couplers of the same number as its input points. The photo-couplers are used for isolating the internal circuit of the programmable controller from the influence of input signals. FIG. 7 shows a typical input circuit using a photo-coupler 3 for isolation. This circuit comprises input terminals TO and COM, a diode bridge 1 having input nodes which are connected to the input terminals TO and COM, a light emitting diode 2 and a photo-coupler 3. The photo-coupler 3 comprises a light emitting diode 3a serving as the input end thereof and a photo-transistor 3b disposed adjacent to the light emitting diode 3a and serving as the output end thereof. The two light emitting diodes 2 and 3a are connected in series across the output nodes of the diode bridge 1.
According to this circuit, the input signal supplied across terminals TO and COM is rectified by the diode bridge 1 and the rectified output of the diode bridge 1 is supplied to an internal circuit by way of the uni-directional photo-coupler 3 for isolation while the presence of the input signal is displayed to the outside by the light emitting diode 2 which is connected in series with the other light emitting diode 3a serving as the input end of the photo-coupler 3. By use of the diode bridge 1, the input signal of this input circuit may be a DC signal of either polarity or even an AC signal, and not only the signal is transferred across the photo-coupler 3 but also the presence of the input signal is indicated by the light emitting diode 2 irrespective of the nature of the input signal.
However, according to this circuit, since the diode bridge 1 is relatively expensive and occupies a substantial area on the circuit board, the cost of the input circuit tends to be high and the size of the input circuit can not be reduced to the desired extent.
FIG. 8 is a circuit diagram of another example of an input circuit which is used in an input unit of a conventional programmable controller. In this circuit, the input signal applied across terminals TO and COM is transferred to an internal circuit directly by way of a bi-directional input type photo-coupler 4 having a pair of light emitting diodes 4a and 4b which are connected in parallel to each other but directionally opposite and the presence of the input signal is indicated by a light emitting diode 5 which is connected in series with an output element of the photo-coupler 4 which is a photo-transistor 4c in this case.
According to this circuit, the cost and size reduction of the input unit is possible through elimination of a diode bridge, but since the light emitting diode 5 receives electric power from the internal circuit, the indication of the presence of an input signal may not be accurate since the function of the light emitting diode 5 is affected by the condition of the internal circuit.
Thus, as shown in FIG. 9, it is conceivable to improve the circuit of FIG. 8 by connecting the light emitting diode 6 in series with the input side of the bi-directional input type photo-coupler 4, but if the input is a DC signal there is a restriction on the polarity [potential] of terminal TO and a negative input is unacceptable.